There are known alloys that contain the specified elements (RF patents No. 2283889 and No. 2169782). Invention of these alloys has been preconditioned by the current trends to increase weight-and-size characteristics of commercial airplanes, which resulted in the increase of sections of highly loaded components such as landing gears. At the same time material requirements has become more strict enforcing good combination of high tensile strength and high impact strength. These structural components are made either of high-alloyed steels or titanium alloys. Substitution of titanium alloys for high-alloyed steels is potentially very advantageous, it helps to achieve at least 1.5 times reduction of component's weight, minimize corrosion and functional problems. However, despite beneficial specific strength behavior of titanium alloys as compared with steel, their use is limited by processing capabilities, in particular, difficulties with uniform mechanical properties for sections sizes exceeding 3 inches in thickness. The said alloys overcome this conflict and can be used to manufacture a wide range of critical components including large forgings and die forgings with section sizes over 150-200 mm and also small semi-products, such as bar, plate with thickness up to 75 mm, which are widely used for the aircraft application including fastener application.
The available methods of melting of homogeneous ingots containing high amounts of refractory β stabilizers, which are characteristic of these alloys, do not meet current requirements to the full extent.
It is well known, that α+β alloy containing 7% aluminum and 4% molybdenum with balance titanium can be easily produced with homogeneous chemistry by melting Al—Mo master alloys and titanium sponge. There are also widely known similar double and triple master alloys, such as Al—V, Al—Sn, Al—Mo—Ti and Al—Cr—Mo, which can be used together with pure metals, as applicable, to melt any low- and medium-alloyed titanium alloys (“Melting and casting of titanium alloys”, A. L. Andreyev, N. F. Anoshkin et al., M., Metallurgy, 1994, pg. 127, table 20 [1]).
However, these and similar master alloys cannot be used for melting of high-alloyed alloys with the relatively low (5%) content of aluminum and high content of refractory, strongly segregating and volatile elements (Mo, V, Cr, Fe, Zr).
There is a known master alloy (RF patent No. 2238344, IPC C22C21/00, C22C1/03) used for melting of titanium alloys, which contains aluminum, vanadium, molybdenum, iron, silicon, chromium, zirconium, oxygen, carbon and nitrogen in the following weight percentages:
Vanadium 26-35
Molybdenum 26-35
Chromium 13-20
Iron 0.1-0.5
Zirconium 0.05-6.0
Silicon 0.35 max.
Each element in the group
containing Oxygen,
Carbon and Nitrogen 0.2 max.
Aluminum balance.
Pilot ingot heats melted (double vacuum-arc remelt (VAR)) using similar master alloy enabled production of high-alloyed titanium alloys with controlled content of aluminum and high chemical homogeneity of the ingot.
Comprehensive mechanical testing of melted alloys revealed instability of properties and relatively low impact strength, which is detrimental to commercial value of these alloys and prevents their application in the aerospace sector.
The major root cause of the above is formation of thin oxide layers at the boundaries of matrix grain, which is the result of presence of oxygen in master alloy constituents and also of silicon, but to a considerably lesser extent, which deteriorates ductility.
There is a known method for melting of titanium alloy ingots, which includes master alloy preparation, weighing, blending and portion-by-portion compaction of solid and loose constituents comprising titanium sponge, master alloy and recyclable scrap to make a consumable electrode for its subsequent double vacuum-arc remelting or a single scull melting followed by a single vacuum-arc remelting (“Melting and casting of titanium alloys”, A. L. Andreyev et al., M., Metallurgy, 1994, pgs. 125-128, 188-230)—prototype.
The known method has a certain drawback, i.e. the introduction of refractory alloying elements in the form of pure metals during melting of titanium alloys (molybdenum in particular), no matter how finely crushed they are, might lead to inclusions that can survive even the second remelt. That is why these elements are introduced in the form of intermediate alloys—master alloys. Manufacture of such master alloys for commercial melting of titanium alloys is cost effective only when done by aluminothermic process. Here a complex master alloy contains considerable amounts of oxygen, which adds to oxygen in other components of the blend and also in the residual atmosphere of vacuum-arc furnace, which leads to critical deterioration of mechanical behavior of titanium alloy. Oxygen is absorbed by titanium and promotes formation of interstitial structures at the grain boundaries having high strength, hardness (maybe twice as high as that of titanium) and low ductility. Specialists are aware of the fact that fracture toughness considerably increases with decreasing oxygen content in titanium matrix.